In a small bowl, slowly whisk the oil into the vinegar. Whisk in the onion and 1 Tbs. finely grated zest from the oranges, and season to taste with salt and pepper.
Cut the peel and white pith from the oranges and cut them in half lengthwise. Cut lengthwise again into 1/3-inch-thick slices. Holding the slices together, cut crosswise into thirds and put them in a large bowl. Add the avocado, spinach, and shrimp to the bowl.
Whisk the vinaigrette and gently toss with the salad. Season to taste with salt and pepper. Sprinkle with the pistachios and serve.
nutrition information (per serving):
Photo: Scott Phillips